1. Field of the Invention
The present invention relates to a semiconductor device of a pin-grid array type. More particularly, it is concerned with a semiconductor of pin-grid array type for surface mounting, and also to a method for mounting the same to a printed circuit board.
2. Description of the Prior Art
A description is given of a pin-grid array (PGA for short hereinafter) which is made up of a semiconductor pellet (pellet for short hereinafter), a substrate on which the pellet is mounted, and a large number of lead pins, in "VLSI Technology" (p. 581) published in 1983 by McGraw-Hill Co., Ltd.
The PGA is conventionally mounted on the printed circuit board by inserting the lead pins of the PGA into through-holes in the printed circuit board. As the number of lead pins of the typical PGA has increased recently, this conventional mounting method has been switched to surface mounting which involves the soldering of the lead pins to the surface of the printed circuit board. The surface mounting of the PGA is typically accomplished by the steps of applying solder cream to the surface of lands (e.g., electrodes) formed on the printed circuit board, placing the package on the printed circuit board, with the tips of the lead pins aligned with the lands, and melting the solder in a reflow furnace, thereby joining the lead pins to the respective lands. A disadvantage of this mounting method is that the lead pins easily get out of position relative to the lands during soldering.
This disadvantage is not so serious in the case of a comparatively light PGA having a relatively small number of pins, because it is positioned correctly on account of the self-aligning effect when the solder reflows. This is not the case with a heavy PGA having a large number of pins which does not permit self-aligning. As a result, the tips of the lead pins get out of the lands during the temporary fitting operation. This causes incomplete connection or continuity between the lead pins and the lands.
The mounting of the PGA to a printed circuit board poses another problem which arises from the fact that the lead pins of the PGA are made slightly soft so that they can absorb strain resulting from an external force applied to the printed circuit board and package or a difference in their coefficient of thermal expansion. Such lead pins are subject to deformation when the PGA is pressed against the printed circuit board for mounting. The deformed lead pins lead to incomplete connection and continuity.